Introduction
Background: sources and pathways for urban groundwater contamination in sub-Saharan Africa
Component | Category | Risk factors |
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Regional considerations | – | Population density |
Land use and land cover | ||
Physical relief/slope | ||
Rainfall amount and intensity | ||
Sources | Municipal and household level sources including domestic livestock and urban agriculture |
Surface sources:
Open defecation from humans and animals Surface waste sites and incineration sites Fertilisers and pesticides and waste use (solid/Liquid) Atmospheric deposition of combustion products |
Sub-surface sources:
Pit latrines Septic tanks Soak-aways Waste pits Cemetery or other burial sites Open sewers/drains—most common type in SSA Reticulated sewers—very limit coverage
Other potential sources:
Market places, abattoir waste, both liquid and solid | ||
Hospital or treatment centre |
Surface and subsurface sources:
Liquid waste discharge to soak-aways/surface channels Solid medical waste disposal Latrines/septic tanks on site | |
Industry e.g. mining |
Surface and subsurface sources:
Process plant effluent Solid waste disposal sites Storage tanks including petroleum products Site runoff and leaching from mine spoil | |
Pathways | Horizontal and vertical pathways in unsaturated and saturated zone |
Shallow sub horizontal pathways in tropical soil: Tropical soils, e.g. Plithosol/Ferrasol horizons present Shallow depth to water table Thin soils and low organic matter content Natural rapid bypass from tree roots and burrows
Vertical and horizontal pathways in saturated zone:
Thin low-permeability zone above weathered basement Thickness and maturity of weathered basement zone Fracture size, length and density in the more competent bedrock below weathered basement |
Local/headwork pathways | Lack of dugwell headwall and/or lining Lack of well cover Use of bucket and rope—soil/animal/human contact Gap between apron and well lining Damaged well apron Propensity for surface flooding Gap between borehole riser/apron Damaged borehole apron Eroded or de-vegetated spring backfill |
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Municipal/domestic waste: for example pit latrines, septic tanks, sewer leakage, sewage effluent, sewage sludge, urban road runoff, landfill/waste dumps and health care facilities
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Industrial sources and waste: for example process waters, plant effluent, stored hydrocarbons, tank and pipeline leakage
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Urban agriculture: for example leached salts, fertilisers, pesticides and animal/human waste
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Mining activities: including both current and historical solid and liquid waste
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Is in direct contact with the headworks of boreholes, wells and springs and where pathways exist that allow this to mix with the water supplies
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Has infiltrated into the sub-surface in the close vicinity of a borehole, well or spring moves along fast horizontal pathways to the supply
Data and methods
Literature review of groundwater quality in urban SSA
Primary term | Term description | Selected search terms |
---|---|---|
Population | ||
Groundwater | Environment, source/pathway of interest | Groundwater; aquifer; well; borehole, spring |
Location/landuse | Geographical area of interest | Africa; individual countries; cities; towns |
Outcome | ||
Chemical/physical contamination | Parameters of interest | Contamination; pollution; water quality; nitrate |
Microbiological contamination | Organisms or indicator organisms of interest | Faecal/faecal contaminant/coliform; thermotolerant coliforms (TTC); microbe; pathogen; enteric; E. coli; virus |
Quantitative assessment of intrinsic aquifer vulnerability index
Results and discussion
Physical, chemical and microbiological indicators of groundwater contamination
Turbidity, total dissolved solids and specific electrical conductance
Region/country (rural/urban) | Geology/subsurface conditions | Sample sites (n) | Water quality parameters | Sampling time frame | Conclusion | Reference |
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Kulanda town in Bo, Sierra Leone (urban) | Weathered granitic basement | Wells (33), lined and unlined | FC, SEC, NO3, Turb, inorganic majors, pH | Wet season | No statistical significance found for pit latrine distance, lowest P value (0.06) for distance from field. Low pH concern for corrosion. | Jimmy et al. (2013) |
Kamangira, Zimbabwe (rural) | Sandy soils over fractured basement | Installed test wells (17) | NH4, NO3, turb, pH, Conductivity, TC, FC | Feb–May 2005 | Low FC >5 m from PL, N conc. usually below WHO standards | Dzwairo et al. (2006) |
Epworth, Zimbabwe (urban) | Fine sandy soils over fractured basement | New and existing wells (18) and boreholes (10) | Na, Zn, Cu, Fe, PO4, NO2, TC, FC | N/A | Elevated N and coliforms in most of the study area | Zingoni et al. (2005) |
Epworth, Zimbabwe (urban) | Fine sandy soils over fractured basement | Installed wells | N, SO4, FC | 2-8 week intervals 1998–1999 | Rapid reduction in coliforms, S and N 5–20 m from PL | Chidavaenzi et al. (2000) |
Lusaka, Zambia (urban) | Thin soils and karstic dolomite | Existing wells (NA) | NO3, Cl, FC | November 2003, March 2004, October 2004 | Greatest FC loading from PL and other waste sources in wet season and dilution of N pollution | Nkhuwa (2006) |
Kabwe, Zambia (urban) | Thick overburden and karstic dolomite | Existing wells and boreholes (75) | TTC, NO3, Cl | Wet and dry seasons: Sep 2013 and Jan 2014 | Greatest TTC and NO3 in shallow wells. Significantly better water quality in boreholes. Higher TTCs in the wet season compared to dry season | Sorensen et al. (2015b) |
Dakar, Senegal (urban) | Fine-course sands over sediments | Existing wells (47) | Broad hydrochemistry, FC | July and November 1989 | Nitrate strongly linked to PL proximity | Tandia et al. (1999) |
NW Province, South Africa (rural) | N/A | Existing wells (9) | NH4, NO3, NO2
| June–July | High contamination <11 m from PL | Vinger et al. (2012) |
Mbazwana, South Africa (urban) | Sands | Installed test wells (5) | FC and NO3
| Bimonthly 2000–2002 | Low nitrate (<10 mg/L) and FC (<10/100 ml) >1 m from PL | Still and Nash (2002) |
Bostwana, Mochudi/Ramotswa (rural) | Well–poorly drained soils | Existing wells (>60) | P, N, stable isotopes and Cl | N/A | Variable N leaching from PL | Lagerstedt et al. (1994) |
Botswana (rural) | Fractured basement | Existing well and observation well (2) | Broad hydrochemistry, E. coli
| October–February 1977 | Contamination of wells near latrine with E. coli and nitrate | Lewis et al. (1980) |
Various, Benin (rural) | N/A | Existing wells (225) | Andenovirus, rotavirus | Wet/dry season 2003–2007 | Viral contamination is linked to PL proximity | Verheyen et al. (2009) |
Langas, Kenya (urban) | N/A | Existing wells (35) | TC, FC | January–June 1999 | 97% wells positive for FC, 40% of wells >15 m from PL | Kimani-Murage and Ngindu (2007) |
Kisumu, Kenya (urban) | Sedimentary | Existing wells (191) | FC, NO3, Cl | 1998 to 2004 | Density of PL within a 100 m radius was significantly correlated with nitrate and Cl but not FC (PC) | Wright et al. (2013) |
South Lunzu, Blantyre, Malawi (urban) | Weathered basement | Borehole, springs and dug well (4) | SEC, Cl, Fe, FC, FS | Wet and dry season on two occasions | Groundwaters highly contaminated due to poor sanitation and domestic waste disposal. 58% of residence use traditional PL | Palamuleni (2002) |
Uganda, Kampala (urban) | Weathered basement | Piezometers (10) | NO3, Cl, PO4
| March–August 2010 biweekly sampling | PL found to be a significant source of nutrients (N) compared to waste dump | Nyenje et al. (2013) |
Uganda, Kampala (urban) | Weathered basement | Installed wells and spring (17) | SEC, pH, P, NO3, Cl, FC and FS | March–August 2003, weekly and monthly | Widespread well contamination linked to PL and other waste sources | Kulabako et al. (2007) |
Uganda, Kampala (urban) | Weathered basement | Springs (4) | FC, FS, NO3, NH4
| Wet and dry season for 5 consecutive weeks | Widespread contamination from PL and poor animal husbandry, both protected and unprotected sources unfit for drinking | Nsubuga et al. (2004) |
Uganda, Kampala (urban) | Weathered basement | Springs (25) | FC, FS | Monthly September 1998–March 1999 | Spring contamination linked to local environmental hygiene and completion rather than on-site sanitation (LR) | Howard et al. (2003) |
Lichinga, Mozambique (urban) | Mudstone | Lichinga (25) | TTC, EF (Enterococi) | Monthly for 1 year | Higher risk at onset of the wet season and end of the dry season. Predominant source was from animal faeces rather than PL or septic tanks (LR) | Godfrey et al. (2006) |
Nitrate and chloride
Ammonium and phosphate
Trace elements including heavy metals
Area/country | Geology | Sample sites | Results (mg/L), range and/or mean where available | Sources | Reference | |
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Ojota, Nigeria | Sedimentary | 10 boreholes, 10 dug wells | SEC 68–3030, mean 584 μS/cm Fe 0–21.4, mean 4.23 Cu 0–33, mean 0.02 Pb 0–14.8, mean 2.4 Zn 0–0.23, mean 0.04 | Industrial areas and landfill, Sites within 2 km radius of landfill affected. | Oyeku and Eludoyin (2010) | |
Akure, Nigeria | Basement complex | Boreholes in landfill vicinity | TDS 18–342 NO3 30–61 Fe 0.9–1.4 Pb 0–1.21 Zn 0–2.3 Cr 0–0.25 | Landfill values decrease with distance 50–100 m | Akinbile and Yusoff (2011) | |
Igando, Lagos, Nigeria | Sedimentary | Wells 10–375 m from landfill | TDS 3–23, mean 9.0 NO3 17.4–60.5, mean 38.5 NH4 0.12–0.3, mean 0.22 PO4 7.07–15.12, mean 10.7 | Municipal landfill | Longe and Balogun (2009) | |
Ibadan, Nigeria | Basement complex | Soil and groundwater | Cd 0.01 Cr, Pb, Co, Ni not detected | Municipal refuse dumps | Adelekan and Alawode (2011) | |
Ilorin, Nigeria | Basement complex | Colour, turbidity over WHO limit
E. coli 161–731 TC 1600–>1,800 | Industrial estate | Adekunle (2009) | ||
Lokpaukwu, Lekwesi and Ishiagu, Nigeria | Shales and igneous intrusions | Springs and open dug wells |
Dry season
TDS 25–3,150 Cl 0–30 NO3–N 0.04–0.74 SO4 0–33.6 Fe 0–3.98 mg/L Mn 0–0.21 mg/L Pb BDL Zn 0–0.06 mg/L Cd 0–0.258 mg/L |
Wet season
TDS 33–11,126 Cl 2.1–1155 NO3–N 0.04–0.68 SO4 1–381 Fe 0–5.07 mg/L Mn 0–0.82 mg/L Pb 0–0.24 mg/L Zn 0–1.07 mg/L Cd 0–0.196 mg/L | Mining | Ezekwe et al. (2012) |
Dar-es-Salaam, Tanzania | Sedimentary | Wells up and down gradient |
Dry up gradient
Mn 0.03 Fe 0.07 FC (cfu x 104/100 ml) 1.5
E. coli 5400 SO4 76
Dry down gradient
Mn 0.02 Fe 0.12 FC (cfu x 104/100 ml) 3.4
E. coli 6500 SO4 49 |
Wet up gradient
Mn 0.00 Fe 0.12 FC (cfu x 104/100 ml) 0.7
E. coli 5000 SO4 35
Wet down gradient
Mn 0.05 Fe 0.24 FC (cfu x 104/100 ml) 3.7
E. coli 5300 SO4 72 | Solid waste disposal | Kassenga and Mbuligwe (2009) |
Lusaka and Copperbelt, Zambia | Dolomites | Surface and groundwater | As 0–0.506, mean 0.009 Cr 0–0.089, mean 0.01 Cu 0–0.270, mean 0.012 Mn 0–10.4, mean 0.369 Ni 0–0.698, mean 0.015 Pb 0–0.094, mean 0.003 Zn 0–1.21, mean 0.75 | Mining: Mn, Cu and Ni correlated | Nachiyunde et al. (2013a) |
Micro-organic pollutants
Pathogens and microbiological indicators
Impacts from in-situ sanitation
Microbiological contaminants
Chemical contaminants
Impacts from non-sanitary anthropogenic sources
Seasonal trends in groundwater quality and implications for climate change
Comparisons between different groundwater source types
Town/city/area | Country | Geology/sites | Water quality (cfu/100 ml) | Contamination | Reference |
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Oju area | Nigeria | Sedimentary
n = 30 | Borehole: FC BDL–500, typically <200. Improved well: FC 50–500, typically >200. Traditional well: FC > 500 | Borehole < improved well < traditional well | Bonsor et al. (2014) |
Yaounde | Cameroon | Basement
n = 40 | Spring: FC 2–72, FS 0. Well: FC 7–100, FS 0–100 | Spring < well | Ewodo et al. (2009) |
Kumasi | Ghana | Basement
n = 9 | Well: FC mean >30,000, EnC = 0–1,152. Borehole: FC mean > 20,000, EnC 0–36 | Borehole < well | Obiri-Danso et al. (2009) |
Blantyre | Malawi | Basement
n = 9 | Borehole: FC 0–30, FS 0. Spring: FC 530–9,500, FS 0–7,000. Wells: FC 3500–11,000, FS 250–2,650 | Borehole < spring < well | Palamuleni (2002) |
Njala | Sierra Leone | Basement
n = 8 | Spring: FC 50–30,000, FS 8–2500. Wells: FC 125–63,000, FS 5–2,500 | Spring < well | Wright (1986) |
Kampala | Uganda | Basement
n = 16 | Spring: FC 29–10,000, FS 6-8300. Wells: FC 0–266, FS 0–268
| Spring < wells | Kulabako et al. (2007) |
Harare | Zimbabwe | Basement
n = 29 | Borehole: FC 0–30,000. Well: FC 0–30,000 | Borehole < well for FC | Zingoni et al. (2005) |
Douala | Cameroon | Sedimentary
n = 4 | Spring: FC 1–950, FS 0–420. Borehole: FC 1–2,300, FS 0–1,400 | Spring < borehole | Takem et al. (2010) |
Kabwe | Zambia | Karstic
n = 75 | Borehole: FC<2–630. Well: FC <2–28,000 | Borehole < well | Sorensen et al. (2015b) |
Separation between sources of pollution and groundwater abstraction points
Assessing urban groundwater pollution risk and nitrate concentrations in SSA
Variable | Coefficient (95% confidence intervals) |
P value |
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Aquifer vulnerability (DRASTIC) risk score | 0.085 (0.077–0.093) | <0.001 |
Population density per ha | −0.00018 (−0.0015–0.0012) | 0.79 |
Proportion of boreholes sampled | −0.105 (−0.346–0.136) | 0.393 |
Constant | −13.374 (−14.835 to −11.913) | <0.001 |
Conclusions and research priorities
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There is a paucity of high-quality studies and limited systematic monitoring data for groundwater quality in SSA. More systematic studies, including those with larger populations and randomized designs and therefore more generalizable, as well as focussed high-frequency temporal studies, are needed to provide a better evidence base from which to make recommendations for groundwater protection and improved health outcomes in SSA.
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Sources of faecal contamination, many of which can by-pass natural attenuation mechanisms, are widespread in most urban centres in SSA, posing a continued threat to shallow groundwater supplies and users.
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High-intensity rainfall events pose a risk to shallow and poorly protected groundwater sources, this is particularly an issue for unimproved dug wells and springs. Shallow groundwater levels also pose a significant risk to shallow water sources in urban settings due to reduced attenuation in the unsaturated zone when groundwater intersects with the base of pit latrines and sewer networks.
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Borehole sources, and the deeper (e.g. > 40 mbgl) groundwater resources they often access, are of generally much better chemical and microbiological status compared to shallow groundwater sources, and as such these sources need to be protected and managed. However, contamination may still be a major issue for boreholes in karstic or fractured basement settings or where there has been long-term contaminant loading to the subsurface and/or high rates of abstraction.
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Results from the literature suggest that using fixed separation distances between sources and receptors cannot be applied without considering the detailed hydrogeological setting, including an understanding of rapid lateral and vertical pathways that may be present, and assessing risks from diffuse sources of contamination.
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There is some evidence (Table 6) to support a link between DRASTIC scores and mean nitrate levels in urban areas reported in literature. However, this relationship is complex due to a range of factors including the variety of urban and non-urban nitrate sources of pollution as well as hydrogeological N processes that may remove nitrate, as a result there are limited benefits from using this approach to understand urban nitrate pollution in groundwater.